The invention relates to a microwave cavity resonator for use as a measuring cell for process gas analyzers. With such a device, the concentration of a predetermined gas component in a process gas is determined by microwave spectroscopic methods utilizing the Stark effect. Usually, the measurements are performed according to the penetration radiation principle, wherein spatially very long measuring cells are used (see DE 36 45 240 C2) in order to obtain a high detection sensitivity.
If such large lengths are to be avoided, resonators can be used as an alternative. In order to be able to detect very low gas concentrations in a highly sensitive way, the Q-factor of the resonator should be high. However, a high Q-factor, that is, a high sensitivity of the resonator also causes the resonator to be sensitive to ambient influences. These influences must be neutralized by stabilizing measures (see in this respect R. Reinschlussel, et al., "Design of a Sensitive Cavity . . . , in AEU, volume 40, 1986, issue No.5, pages 313-320). The compensation for the external influences results in additional construction expenses which substantially increases building costs.
An operative gas analyzer is presented in DE-Z: Technisches Messen 58(1991) 11, pages 433 to 438. Herein a microwave process analyzer for the rapid analysis of for example exhaust gases is described.
U.S. Pat. No. 3,973,186 discloses a microwave hollow resonator for continuous spectroscopic gas analysis. The resonator has a rectangular shape but it may also be cylindrical. The resonance frequency of the resonator can be changed so that various polar molecules can be individually examined.
The object of the invention is derived from the fields of power generation and waste combustion, where the concentrations of ammonia in the exhaust gas should be determined on a real-time basis and without sensitivity to other compounds of the process; that is, the exhaust gases. For this purposes, a analysis apparatus capable of withstanding the environment must be installed in the exhaust gas discharge system which senses the concentration of the particular gas content or gas components reliably over a long period of operation.